Traveling-wave antenna for the current drive using the fast wave in the lower hybrid-frequency range in the Versatile Experiment Spherical Torus (VEST)

Abstract

An antenna using the fast wave in the lower hybrid-resonance frequency range is proposed for the significant attainment of the heating and current-drive efficiencies. For the operational stability regarding the change of the plasma load, the proposed antenna is designed as the combline-structure traveling-wave antenna. It consists of two tapered coaxial cables, two exciting straps of the folded-monopole structure, 12 radiating straps, 11 field isolators, and the Faraday shield. Regarding the vacuum load, the simulated and measured results show the wide impedance bandwidth (45 MHz, VSWR < 2) of the proposed antenna in conjunction with the traveling-wave antenna structure and the folded-monopole exciting strap. Regarding the plasma load, the simulated results show that the proposed antenna provides the impedance bandwidth (35 MHz, VSWR < 2), and the fast wave is coupled with an input power from 51.5 - 90%. The measured and simulated coupling efficiencies are highly compatible in the Versatile Experiment Spherical Torus (VEST). According to the frequency-range changes, the simulated-variable range of the parallel refractive index (eta(parallel to)) is from 3 - 4.5 in the operational frequency band of 475-510 MHz.